Is it permissible to pray Salat al-Layl or other recommended prayers if I have a lot of Qadha prayers? Can I pray Fajr Qadha and still get reward for Salat al-Layl?
Yes. Recommended Prayers especially Salat al-Layl ( Night prayer) is highly recommended for every one including those who have unfulfilled Qadha prayers on them. No doubt, the reward of performing Qadha prayers is much more than the reward of recommended prayers.
You must perform every prayer within its time and never miss any obligatory prayer.
Wassalam.